Electronic cameras are well known in the prior art and include conventional video cameras and digital cameras. These cameras typically include pixel image sensors having rectangular arrays of photodetector picture elements, or pixels, which are used to record conventional images for viewing by human observers. The pixel image sensors required by these cameras are manufactured in large quantities and, as a result, are inexpensive and readily available for use by consumers.
Special purpose electronic cameras, that is, electronic cameras that are designed to perform specific functions rather than to simply allow a consumer to view conventional images, are also well known in the prior art. These cameras are used in a variety of fields, including the security, manufacturing, and inspection fields, and include line scan cameras, rosette scan cameras, quadrant detectors, pan-zoom-tilt cameras, cameras that trade off numbers of pixels for frame rate, stereo cameras, windowing cameras, lock-in cameras (experimental only), and others. In contrast to other electronic cameras, special purpose cameras typically include custom designed detector element image sensors, i.e., image sensors that include detector elements having various shapes and sizes rather than pixels.
Prior art special purpose cameras currently have a number of disadvantages that make them undesirable in many applications. First, the custom designed detector element image sensors required by these cameras are generally manufactured in small quantities for specific applications and tend to be expensive. This causes these types of cameras to be expensive.
Second, these cameras tend to be large and bulky because the custom designed detector element image sensors usually require amplifiers for each detector element that are physically separated from the detector elements and connected using separate cables. Examples of special purpose cameras having this disadvantage are described in U.S. Pat. No. 3,689,772 (“the '772 patent”), issued to George et al. on Sep. 5, 1972 and entitled “Photodetector Light Pattern Detector” and U.S. Pat. No. 4,869,574, issued to Hartman on Sep. 26, 1989 and entitled “Hybrid Optical Correlator.”
The problem of large and bulky special purpose cameras has been partially addressed by one of the inventors listed on the '772 patent, Nicholas George, who developed a special purpose camera that included a hybrid ring/wedge detector element image sensor, a digital multiplexer for acquiring and relaying data to a host computer, and an electronic system that grouped the amplifiers required by the detector element image sensor onto a series of electronic boards. This camera, however, still required separate cables for each ring and wedge shaped detector element in the image sensor.
Special purpose electronic cameras have limited capabilities because they include custom designed detector element image sensors that can only be used to perform certain specific functions. For example, some prior art special purpose cameras include image sensors that include ring shaped detector elements that can be used to analyze optical power spectra, while others include image sensors that include wedge shaped detector elements that can be used to make precise angular measurements. Image sensors including ring shaped detector elements cannot be used to perform the same functions that are performed by image sensors including wedge shaped detector elements.
This problem has been partially addressed by the prior art, which teaches the use of special purpose cameras that include hybrid or combination detector element image sensors. An example of one such image sensor is described in the '772 patent referenced above. The hybrid ring/wedge detector element image sensor taught by the '772 patent includes both ring shaped and wedge shaped detector elements and can be used to analyze optical power spectra and to make precise angular measurements.
While hybrid detector element image sensors can be used to increase the capabilities of special purpose cameras, they may not be practical in some applications. For example, hybrid image sensors may be too expensive or too large to be used in a given application. Furthermore, even where hybrid image sensors can be used, special purpose cameras are still limited to performing only the functions that can be performed using the hybrid image sensor.
The limited capability problem has also been addressed by the assignee of the present invention, Advanced Optical Systems, Inc., who developed a special purpose camera using a combination detector element image sensor that includes two sub arrays: a first sub array including multiple ring and wedge shaped detector elements for analyzing optical power spectra, and a second sub array including multiple off-set wedge shaped detector elements for precise angular measurement. This combination detector element image sensor, however, has the same disadvantages associated with the hybrid ring/wedge image sensor discussed above, as well as several additional disadvantages.
First, it is inherently expensive because it is a special purpose image sensor and has limited application. Second, although both sub arrays are present on a single detector chip, only one sub array can be used at any given time. Third, each detector element requires a dual-stage amplifier, which doubles the number of amplifiers required by this image sensor and increases the size of the circuit board required to support the image sensor. The large number of amplifiers also generates a great deal of heat. Finally, this image sensor requires short leads between the detector elements and the amplifiers and, as a result, the image sensor has to be mounted directly on the circuit board with the amplifiers. This, in turn, makes it awkward to mount the optical system used with the image sensor. This prior art camera is described in a paper written by the inventors listed on the present application and entitled “Fourier Plane Detectors for Optical Image Processing.” That paper is hereby incorporated by reference into this application.
An additional disadvantage associated with special purpose cameras is the fact that the capabilities of these cameras typically cannot be modified once they are purchased by a consumer. If a consumer desires to do so, he or she typically must purchase an entirely new special purpose camera with the desired capabilities. This increases costs for consumers who require a special purpose camera that can perform multiple different functions.
One example of this type of special purpose camera is the circular scan camera. This type of camera includes a detector element image sensor having annular shaped detector elements of a predetermined height, distance from the center, and angular width. In order to change the height, distance from center, or angular width of the image sensor, one must purchase a new camera having a new specially designed detector element image sensor having detector elements with the required height, distance from center, and angular width.
Another problem with special purpose electronic cameras is that they are difficult to manufacture because the custom designed detector element image sensors used in these devices are not readily available from manufacturers. This is due primarily to the fact that these types of image sensors are manufactured in small quantities and are expensive.
The assignee of the present invention is aware of one prior art special purpose camera that does not require a custom designed detector element image sensor. This camera is described in a paper written by D. J. F. Walker and C. R. Chatwin and entitled “Fast Object Recognition using a Hybrid Optical/Digital Processor.” This paper was published in the September 1990 Proceedings of the British Machine Vision Conference, pgs. 413–418, RAE Cat: 4, CVCP Cat: 4, and is hereby incorporated by reference into this application. The prior art camera described in this paper includes a CCD pixel image sensor including a pixel array, external analog to digital converters, and a digital signal processor that is used to form wedge and ring samples using individual pixels included in the pixel array.
Although this prior art camera does not require the use of a custom designed detector element image sensor and, as a result, does not have the disadvantages associated with this type of requirement, it nevertheless still has several of the disadvantages discussed above with regard to other prior art special purpose cameras. This camera is large and bulky and can only be used to perform the functions that can be performed using the wedge and ring samples. This camera cannot be easily modified to perform additional functions. This camera also has the additional disadvantage that it generates ring and wedge samples that include blank areas because pixels along the edges of a wedge or ring are eliminated from consideration.
What is needed, then, is a special purpose camera that is smaller, more compact, and less expensive, and that can be used to perform multiple different functions, that can be easily modified to perform one function and then another, and that includes an image sensor that is inexpensive and readily available from manufacturers.